For such a long challenge walk, all on hard impact tarmac, the best footwear would be comfortable, well cushioned trainers, with high quality sports socks, like a twin skin 1000miler.
Signing up, or entering such a long challenge event, on a whim, is most unwise, and it would be interesting to hear from him, how far he managed to travel, i guess around 35-40miles.
My past experience on such long challenge distances, was for a first timer, to be excited and full of trepidation, but choosing such a mammoth distance, on such a hard surface,is unwise.
I recon the challenge was a step too far, but saying that, i did accomplish a tough 54miler across the Brecon Beacons, being green and a total novice, gun ho, up for a challenge, entering a long walk for the fun of it.
That was after several years, of regular training over the same terrain, covering shorter distances of around twenty to thirty miles, but i can remember how exhausted i felt after completing the walk.
Years ago, tromping along hard surfaces wearing sometimes heavy cumbersome boots was murder on the legs., especially when you pass the 50-60mile barrier, its then mind of matter, sheer torture until you reach the end.
Whilst attempting my various LDWA 100, once your body has passed the 50mile barrier, your so tired, that the only thing keeping you going, was your endurance and ability to keep moving whilst your body told you to rest.
The further you went, the more tired and fatigued you became, with the danger, that if you had any self doubt about your ability to continue, something in your head, would tell you to pull out, quit, and the urge to do so, when so tired, out on your legs, was huge.
When your 100% sure of the terrain you will be covering, choice of footwear is crucial, especially on mile after relentless mile of concrete and tarmac.
You need very comfortable, lightweight footwear, because after all, the more weight on your legs, the more strain and eventual tiredness you will experience later in the walk, and the London to Brighton is a long one.
If the Challenge Walk was on a mixture of mountains and main roads, then maybe a pair of boots would be the order of the day, but for tarmac, comfortable trainers.